An atomic clock is an oscillator that provides unmatched frequency stability over long periods of time because their resonance frequency is determined by the energy transition of the atoms, in contrast to crystal oscillators, where the frequency is determined by the length of the crystal and is therefore much more susceptible to temperature variations.
Atomic clocks are utilized in various systems which require extremely accurate and stable frequencies, such as in bistatic radars, GPS (global positioning system) and other navigation and positioning systems, as well as in communications systems, cellular phone systems and scientific experiments, by way of example.
In one type of atomic clock, a cell containing an active medium such as cesium (or rubidium) vapor is irradiated with optical energy whereby light from an optical source pumps the atoms of the vapor from a ground state to a higher state from which they fall to a state which is at a hyperfine wavelength above the ground state. In this manner a controlled amount of the light is propagated through the cell and is detected by means of a photodetector.
An optical pumping means, such as a laser diode is operable to transmit a light beam of a particular wavelength through the active vapor, which is excited to a higher state. Absorption of the light in pumping the atoms of the vapor to the higher states is sensed by a photodetector which provides an output signal proportional to the impinging light beam on the detector.
By examining the output of the photodetector, a control means provides various control signals to ensure that the wavelength of the propagated light is precisely controlled.
In operation, alkali metal deposits have a tendency to condense at the center of the top glass plate of the alkali cell just below the photodetector, thus causing significant signal loss due to reduced light transmission. There is a need for a method of reducing or eliminating the metal deposits on the center of the top glass plate of the alkali cell.